Numerous polymer-based medical devices have been developed for implantation or insertion into the body. For example, various state of the art medical devices consist of a medical device substrate with a polymeric coating that serves as a reservoir for one or more therapeutic agents. Specific examples include drug eluting coronary stents, commercially available from Boston Scientific Corp. (TAXUS), Johnson & Johnson (CYPHER) and others, which have become the standard of care for maintaining vessel patency after balloon angioplasty. These products are based on metallic balloon expandable stents with polymeric coatings that release antiproliferative drugs at a controlled rate and total dose effective to inhibit the smooth muscle proliferation that is associated with restenosis (vessel reclosure).
Various types of polymeric materials have been used as drug-releasing reservoirs, including, for example, homopolymers such as poly(n-butyl methacrylate) and copolymers such as poly(isobutylene-co-styrene), for example, poly(styrene-b-isobutylene-b-styrene) triblock copolymers (SIBS), which are described, for instance, in U.S. Pat. No. 6,545,097 to Pinchuk et al. In addition to their utility as drug delivery reservoirs, SIBS copolymers have proven valuable for a variety of reasons, including their excellent biocompatibility, elasticity, strength, and processability. The latter characteristics are due, at least in part, to the fact that SIBS copolymers are thermoplastic elastomers. Thermoplastic elastomers are elastomeric (i.e., reversibly deformable) polymers that form physical crosslinks which can be reversed, for example, by dissolving or melting the polymer. SIBS triblock copolymers have an elastomeric low glass transition temperature (Tg) midblock and hard elevated Tg endblocks. As with many block copolymers, SIBS tends to phase separate, with the elastomeric blocks aggregating to form elastomeric phase domains and the hard blocks aggregating to form hard phase domains. It has been hypothesized that, because each elastomeric block has a hard block at each end, and because different hard blocks within the same triblock copolymer are capable of occupying two different hard phase domains, the hard phase domains become physically crosslinked to one another via the soft blocks.